Patients with autism exhibit altered cognitive function that is revealed in EEG recordings as an imbalance of excitation to inhibition in the cerebral cortex. Imbalanced excitation / inhibition could be the product of multiple alterations of brain structure and function, such as an altered ratio of pyramidal neurons to interneurons in the autistic cerebral cortex. Imbalanced excitation / inhibition could also be produced by an alteration in the number of non-neuronal cell types (such as oligodendrocytes or astrocytes) that participate in neurotransmission; and / or by altered morphology or density of neuronal dendrites and dendritic spines in the cerebral cortex. Existing data on the relative number of specific cell types in the cerebral cortex of human autistic brains is at this point insufficient, as is data on the morphology of neurons in the human autistic cerebral cortex. The number of published studies is scarce, the number of cases included in the published studies is small, and the cases used for analysis is heterogeneous. A full understanding of the cellular basis of autism in human brain is essential for establishing the basic underlying mechanisms that, in turn, will result in therapeutic interventions. This project will analyze the temporal lobe of 21 autistic postmortem human brains (diagnosed as autistic, not as other autism spectrum disorders) and the temporal lobe of 18 control postmortem human brains, and investigate: 1) Density of total neurons, pyramidal neurons, and interneuronal subtypes in the autistic versus control temporal cortex using unbiased stereological methods for cell quantification. 2) Density of total glial cells, astrocytes, and oligodendrocytes in the autistic versus control temporal cortex using unbiased stereological methods. Cellular classification will be based on cellular morphology in Nissl stained sections, and on cellular marker expression in immunohistochemical experiments. 3) Dendrite length, dendritic branching, and spine density in the temporal lobe of autistic and control brains. We will stain neurons using the Golgi method, Neurolucida and the Scholl method to determine total dendritic length, total number of dendritic branches and spine density in pyramidal neurons and interneurons. We will classify our cases based on the subject's clinical history, including age, the presence of seizures or mental retardation, and the levels of social, communicative, and repetitive behaviors. We will statistically compare densities in the autistic versus control temporal cortex within clinical categories. A full understanding of the cellular basis of autism in the human brain is essential for establishing underlying mechanisms that in turn could yield new therapeutic interventions. The data obtained from this project will also serve as the basis for future studies focusing on the neuropathology and the imbalance excitation / inhibition of the autistic brain.